To fight Oni in Arcade Mode, you need to never lose a round, have at least one 1 Perfect victory, have 10 First Attacks, complete 5 matches with either Super or Ultra Combos, and then make sure you also defeat Seth with a Super or Ultra Combo as well.

To fight Gouken in Arcade Mode, you have to satisfy these requirements first: Never lose a round, get 1 Perfect victory, get 10 First Attacks, and use Super or Ultra Combos five times to finish matches.